This invention relates to electro-optical devices, and more particularly to an electro-optical device of the type having a controlled laser diode.
For several years now, owing to the advent of compact disks and laser printers, for example, laser diodes have been mass-produced on a large scale and have therefore become very economical. In certain fields of application, they are even increasingly replacing conventional gas lasers owing to their great advantages of miniaturization, long life, efficiency, and ease of execution.
Thus, there are currently a large number of different types of laser diodes, e.g., double-heterojunction diodes, DBR (distributed Bragg reflector) diodes, vertical-cavity diodes, etc., corresponding to a large number of different needs.
Double-heterojunction laser diodes of the AsGaAl type, for instance, find use in a great many different applications. They emit laser light with a wavelength between 750 nm and 880 nm, close to the visible spectrum and adapted to the usual silicon photoelectric receivers.
The light frequency emitted by this type of diodes depends upon two parameters:
the injection current causes the frequency to vary by 3 GHz/mA, or 0.006 nm/mA; PA1 the temperature causes the frequency to vary by 30 GHz/.degree.C., or 0.06 nm/.degree.C.
For many applications, it is necessary to have a laser source emitting at an absolutely stable frequency or wavelength. This is the case for laser diodes used in interferometers, in atomic frequency oscillators, in spectroscopy, or in telecommunications, for example. In such applications, it is therefore necessary to use either an expensive and bulky gas laser or a laser diode having a stabilized emission frequency.
Steady current sources can be designed producing a current of 150 mA with an accuracy of .+-.5 .mu.A. If this current is applied to a laser diode, the frequency inaccuracy will therefore be about df/f=5.multidot.10.sup.-8. Over a long period of time, it is difficult to guarantee a more accurate temperature of the diode than .+-.1 mK. This corresponds to a df/f error of 8.multidot.10.sup.-8.
For some of the previously mentioned applications, the accuracy yielded by the foregoing means proves to be insufficient. Moreover, even if it were possible to maintain a sufficiently constant current and temperature, variations due to aging of the laser diode could not be compensated for with this method. It is therefore necessary to stabilize the laser diode with the aid of an outside reference element.
A. Abou-Zeid, in his article entitled, "Diodenlaser in der industriellen Me.beta.technik," Technische Mitteilungen 85, No. 1, May 1992, pp. 34-43, describes an interferometer using a laser diode stabilized by means of an outside reference element. In FIG. 14 on page 40, the light beam emitted by the laser diode is split by means of a beam splitter into a measuring beam and a control beam. In this article, the measuring beam is used for an interferometry application. The control beam reaches an outside reference element, constituted in FIG. 14 by a Fabry-Perrot etalon and in FIG. 15 by an absorption cell. A sensor measures the intensity of the beam which has passed through the reference element and supplies a signal used by adjusting means to control the current and/or the temperature applied to the laser diode. The reference element is made up in such a way as to attenuate certain wavelengths selectively or, on the contrary, to become resonant when a certain wavelength reaches it. In the design of FIG. 15, the absorption cell contains a gas having an absorption peak situated very precisely at the frequency at which it is desired to lock up the laser diode. The adjusting means control the emission of the diode so that the intensity of the beam passing through the cell is minimal, which amounts to locking up the optical emission frequency of the laser diode to the peak absorption frequency of the cell.
U.S. Pat. No. 5,148,437 describes an atomic frequency oscillator using a laser diode stabilized in a similar manner. In Figure 1b in particular, in element 24, the beam of the laser diode is seen to be separated by means of a beam splitter into a measuring beam and a control beam. The control beam reaches an optical resonator 24d, and a photosensor 24e supplies a signal used by control means 24f, 24g, 24h, 24i to control the current injection of the laser diode so that it emits precisely at the resonance frequency of the optical resonator 24e. In this application, the measuring beam is used to pump atoms optically into a resonance cell 21C.
However, these two solutions have a number of drawbacks making their use problematic. Part of the light beam emitted by the laser diode is reflected by the surface of the beam splitter and returned toward the laser diode. This feedback of light into the laser diode modifies certain emission characteristics, such as the wavelength or the mode of emission, and therefore makes stabilization very difficult. Several solutions have been proposed for reducing, but not eliminating, the undesirable feedback: by treating the faces of the beam splitter with antireflective coatings, by disaligning the beam splitter in relation to the optical axis, or by using an optical insulator, as indicated in the left-hand column on page 37 of the Abou-Zeid reference, for example. Although such operations do improve the device, they entail complications and additional costs.
Furthermore, upon temperature fluctuations, the distance between the laser diode and the interface of the beam splitter changes because of the dilation. The phase and/or amplitude of the light returned to the laser diode therefore depends upon the temperature, making it difficult to predict how the device will perform.
In addition, the positioning of the various components is complicated in these two designs. The absorption cell, which may be quite large in volume according to the chosen gas, must be positioned in the optical path of the light beam, i.e., near the laser diode and the measuring circuit. If the laser diode is placed on a printed circuit, it may be difficult to fix the absorption cell there and to adjust it correctly. Moreover, and particularly in the disclosure of U.S. Pat. No. 5,148,437, the photosensor 24e may be disturbed by the electromagnetic fields caused by the components of the atomic oscillator, especially by the microwave generator. The value measured by this sensor may then be distorted, and the stabilization of the laser diode is affected thereby.
In the two foregoing designs, the beam splitter separates the light beam emitted by the laser diode into two beams--a measuring beam and a controlling beam--of substantially equal intensity. Now, the control means can generally do with a small fraction of the light intensity required by the rest of the electro-optical device. In a typical interferometry application, the control circuit could do with about 2 or 3% of the light intensity required by the interferometer, whereas in these designs it receives the same intensity. The laser diode must therefore operate at a needlessly high power, thus reducing its life span, hastening aging, generating undesirable heating, and wasting energy.
European Patent No. 0 479 118 to Dornier describes a device in which the light beam necessary for controlling a laser diode is taken off by means of an optical fiber placed directly in front of the laser diode. With this solution, the laser diode can be positioned very freely with respect to the rest of the device. However, the latter uses a single-mode optical fiber, difficult to put to work and requiring precautions for the alignment with the laser diode. Moreover, a single-mode fiber is not capable of conveying a light beam of a certain intensity. Hence this device is suitable only for devices in which the the intensity demanded of the operating light beam is low, e.g., in this patent, for fiber optics sensor devices.
Moreover, this solution merely shifts the problems connected with beam splitting remote from the laser diode. In order to effect the separation between the operating light beam and the controlling light beam, it is necessary in this design, too, to provide a beam-splitting element, here in the form of a fiber optics coupler. However, this type of coupler is at least as difficult to produce as a beam splitter. The light entering the prior art couplers passes through an optical interface which returns part of the light toward the laser diode through the optical fiber.
Coupling a single-mode fiber to a Fabry-Perot cavity as is done generates a very great feedback. Hence it is not possible to use the teaching of this patent for applications sensitive to problems of abrupt changes of mode.
Finally, the problem of the power to be furnished to the laser diode is not solved by the above-mentioned patent any more than by the other prior art. Conventional fiber optics couplers in fact split the incoming light beam into two beams of equal intensity. Consequently, the control means receive a light intensity equal to the rest of the device, even though they could do with a fraction of that intensity in many applications.
L. Pujol et al., in a paper entitled "Interferometre integre stabilise par absorption atomique," presented at the OPTICS 92 conference in Paris, describe a laser module stabilized by atomic absorption which avoids some of the drawbacks discussed above. In this reference, the laser diode is a Fabry-Perot laser diode emitting light from both faces. The light emitted by the front face constitutes the operating light beam used for the specific application, here for a device for measuring length by interferometry, whereas the light emitted by the rear face is recovered by an optical fiber and conveyed toward control means which may thus be situated elsewhere. Hence the controlling light beam does not pass through any optical interface between the laser diode and the control means, whereby one of the sources of light feedback in the laser diode can be eliminated.
However, this solution likewise poses a number of problems. For even if, theoretically, all laser diodes having a Fabry-Perot cavity can emit light both from the rear face and from the front face, the majority of the commercially available diodes are optimized and encapsulated so as to emit light only from the front face. Only a limited selection of costly diodes are capable of emitting from both faces.
What is more, placement of the laser diode becomes absolutely critical since its front face must be aligned with the integrated optical device and its rear face with the optical fiber. Both these elements are characterized by a limited optical aperture, thus necessitating careful machining of the laser module. Finally, the light emitted by the laser diode directly enters an integrated optical circuit performing the interferometry function. Some of the light emitted is reflected at the interface of the integrated circuit and returned toward the laser diode. Hence this design does not at all solve the problem of feedback, nor that of the behavior varying with the temperature owing to variations of the index of reflection.
Japanese Patent No. 3,091,283 describes a laser module in which an optical fiber is placed so as to take off part of the light emitted by the laser diode which does not reach the lens. This solution requires an oversize emission cone, hence a waste of power, of the laser diode.
Japanese Patent Publication No. 55 126,208 proposes fixing an optical fiber directly to the surface of the laser diode, even before the lens. This solution is complicated to carry out and is not very suitable for miniaturized laser diodes.
Japanese Patent Publication No. 61 292,977 teaches the recovery by means of an optical fiber of part of the light emitted by the laser diode which reaches the case of the module rather than the lens. Once more, this solution requires an oversize emission cone of the laser diode and therefore a waste of power, causing needless heating of the diode.
German Disclosed Application (DOS) No. 27 41,700 is similar in that it proposes placing the optical fiber in the periphery of the emission cone of the laser diode, only the central area of this cone being used. Hence this solution suffers from the same drawbacks.